bass amp

Behringer Ultrabass BX4500H Bass Head Needs a New Fan

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Billy said that this unit worked very well until it overheated and shut down after about an hour and a half of use.  He also didn’t hear the fan run.  Could something this simple be all that is wrong with this bass head?

The Unbrokenstring Crew operated this unit for an hour and verified that it overheated and shut down, as it should.

 

Here is a tour of the rear panel.  The power switch is in the back of the unit.

 

I really like the Neutrik Speak-On connectors for power at this level.  The 1/4th inch plugs could be operated above their data sheet limits for current and voltage if used at the 450 watt level.

 

We’ve removed the top of the unit.  Removing the finger guard lets us verify that the fan itself is completely locked up.

 

To get to the fan, we need to remove the heat sink assembly, which is held in place with these screws on the bottom.

 

This pic just allows us to keep the wiring straight for reassembly.  This circuit board handles the power output duties.

 

The fan is bolted to the end of the heat sink assembly.  The power amp is also attached to the whole stack.

 

We need these specs in order to specify a replacement.

 

This power cable is specific to the mating connector on the circuit board that supplies the 24v for the fan.  We need to keep this and transfer it to the new fan.

 

There is a chance that we can remove the whole cable assembly from the old fan and move it to the new fan.

 

The new fan has three wires.  The third wire is probably a tachometer output so that the speed of the  fan can be known.

 

The wires on the new fan are soldered to the fan with lap joints.

 

So, we can make new lap joints when moving the old cable to the new fan.  I’ve got a good feeling about this.

 

The new fan is bolted back on the stack and the whole arrangement is put back together.

 

This amp ran for four hours continuously until the neighbors called the cops complaining of a noise ordinance violation.  I believe that the overheating problem is fixed!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Intermittent Ashdown EVO III 500 Bass Head

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This head was soldiering away in the studio when the output signal became distorted.  Can the Unbrokenstring Crew un-distort the output and keep it from happening again?

Stock photo credit: Ashdown Engineering

 

Name, rank and serial number, please.

 

The oscilloscope shows the waveform presented across an eight ohm resistive load.  A sinewave is applied to the input jack.  We should have a sinewave here.  But we don’t.  This gives us something to work on!

 

Oops!  As soon as we touch the chassis, the output waveform changes!

 

Since we’re on a roll, let’s touch it again!  This is what we should have seen all along.  I think we know where to look.

 

You might be surprised to know that the big metal heat sink I was touching has a high voltage on it.  So here I am safely draining the high voltage before I touch it and get shocked.  Like in the previous picture.

 

I wish the heat sink was as well-supported as the rest of the circuit boards.

 

This whole assembly is attached to the circuit board on the bottom.  Out it comes!

 

Can you see the problem?  Me neither.

 

Close examination reveals cracked solder joints.

 

The correct repair for a cracked solder joint is to remove everything and replace the joint with fresh tin/lead solder.

 

This is a good solder joint, if I do say so myself.

 

Further examination reveals more cracked solder joints.  Guess what we’re going to do to these?

 

This little yellow grabber tool is handy to install screws in tight recesses.

 

Time to crank it up!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

 

Peavey Mark III Bass Head – Busted Controls

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These sturdy old bass heads turn up in pawn shops and Craigslist from time to time. They soldier on for years, sometimes making music for decades. This unit came in for some minor repairs and a Million Mile Checkup. Let’s get to work!
This unit has LEDs for the power indicator and a clipping indicator, or something called “Compression.”

 

From the school of ‘crank it up and rip the knobs off’ we have a knob that has been ripped off.

 

Most of these Peavey heads usually consist of a preamp, mixer, and/or EQ assembly behind the front panel, and a rear panel that holds an amplifier and power supply.  The transformer is bolted to the case in the middle.

 

We need to get to the circuit board, so all the knobs and nuts come off.

 

This unit was built in the era of the Plastic Potentiometer Shafts.  Grrr…

 

A few screws keep the panel and circuit board flat.

 

More screws go into the magnetic parts holder.  We are almost there.

 

Here, at last, is the circuit board.

 

Potentiometers that stand off the circuit board like this are sometimes called ‘spider’ pots.

 

These other controls are fine.  The values 10K and 50K refer the resistance, and the letter ‘B’ implies that the taper is linear.  An ‘A’ letter implies an audio taper control.

 

The Alps company made these in Brasil.

 

Here is the new replacement part, with a metal shaft!  If this were my unit, I’d replace them all with metal shafts.

 

The plastic shaft broke off inside this knob.  A few minutes with an Exacto knife is all it took to reclaim the knob and cut my thumb..

 

The new control is soldered into the circuit board.  The factory workmanship on this assembly is pretty good.

 

Back together it goes.

 

With the knobs in place, you can’t tell that anyone has been here.

 

One last look as we reassemble the unit…

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey 1810 Bass Enclosure Crossover Doesn’t (Crossover)

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Most of what you see in this post works well. However, a crossover network inside the speaker cabinet doesn’t work at all. Could the Unbrokenstring Crew sort out a solution that would put this cabinet back to work?
Here is some file old hardware from the 1980s.  This cabinet has an eighteen inch subwoofer underneath two ten inch loudspeakers.

 

Behind this panel is found a crossover network that routes the input signal to the proper destinations.  The crossover automatically routes the really low frequency stuff to the 18 inch loudspeaker and the rest is routed to the ten inch loudspeakers.  Or, you can specify which signal goes to which driver using the BI-AMP jacks.

 

The sheet metal screws hold this circuit board in place.  An inductor plugs into the pins in the center of the picture.

 

The input jacks are seen in this view.  The capacitor is part of the crossover network, and has been replaced.  As this is in a high powered audio network, the electrolytic capacitor is a non-polarized variety.

 

We can examine the circuit side of the printed circuit board to figure out the schematic for this assembly.

 

This inductor is cooked.  Unfortunately, this part is no longer available.

 

An examination of the inductor may give us some clues that we could possibly use to fix it and use it again.

 

This component can take the place of three different inductors, thus the three wires.  Any two wires yield a different inductance.  Unfortunately, the insulation is thoroughly cooked.  No salvaging this guy.

 

The functions of each ‘net’ on this circuit board is labelled with a felt tip marker.

 

For this particular model of crossover, the inductor wires we need to use are indicated by this inductor symbol.

 

This is an inexpensive crossover kit, with similar specifications.  We can harvest this inductor for use in the crossover.

 

Here are a few details, listed on the end of the box.

 

This inductor is has a laminated bar core.

 

We can remove this inductor from the circuit board and use it in the Peavey circuit.

 

The back side of the donor network is covered with some self-adhesive foam rubber.

 

The foam sticks really well!  But now we have access to the solder joints that need to be unsoldered.

 

The mechanical mounting scheme is VERY robust for this heavy part.  We can use all of this in the Peavey circuit.

 

These white plastic caps are handy to hold the mounting nut and to insulate the exposed iron core.

 

We will use the original circuit board as a template for finding and marking places to drill new mounting holes in the Peavey circuit board.  We can search around for a practical mounting location.

 

The mounting holes are marked.  Away We Go!

 

One end of the inductor is electrically wired here.

 

The other end of the inductor is wired here.

 

We will make lock the threads of the mounting screws with this stuff.

 

The thread locking compound is thin enough to seep into the threaded fasteners and lock them.

 

Now, we can reassemble the crossover network assembly.

 

The original inductor was mounted in the foreground.  This doesn’t look to tacky, does it?

 

One last look before it disappears into the enclosure.

 

The panel is ready for reassembly into the enclosure.

 

No.  Wait.  We need a new gasket between the I/O panel and the cabinet enclosure.  This self-adhesive foam strip material is just the ticket for this application.  Note the mitered corners.  Because I’m OCD like that.

 

Holes for the mounting screws are cleared with the Exacto knife.

 

OK, NOW is one last look at this assembly.  The wire pairs go to the loudspeakers in the cabinet.

 

This system is pretty awesome.  Everything tests out at full power.  Life Is Good!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Fender Rumble 350 Bass Combo Repair and Modification

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I can’t help but think that this is not a real Fender, but “Fender Bass Amplification” is the nameplate on this unit, not the cursive Fender logo seen for decades.  This unit is very versatile, works better than most bass combos, but just screams “Designed and Manufactured In China.”

A lot of ‘motor noise’ came from one of these loudspeakers in this Fender Rumble 350 bass combo amp.  The owner said, while we were at it, could we wire this unit so that it could be used as an external cab, driven by another unit?  The UnbrokenString Crew said, ‘Sure!  Why not?’

 

If you listen closely, you can hear the warped voice coil rubbing against the magnet gap inside the bad loudspeaker.

Access to the inside of the cabinet is accomplished by removing the loudspeakers.  We are using an electric screwdriver to drill a pilot hole for a switching Neutrik connector that will allow this unit to be used as an external cabinet.

This Forstner bit is just the right size to clear the body of the connector.

Using the pilot hole, we can cleanly cut through the Tolex and into the wood cabinet.  The scratches in the Tolex were not part of this project.

Yes, this looks like a hole to me.

We will replace both Chinese loudspeakers with a matching pair of 200 watt ea. bass guitar loudspeakers.  The original loudspeakers were rated for 75 watts each, which is strange considering that they were tied to a 350 watt amplifier.  The bass loudspeakers have a different hole pattern, so we are drilling new locations for the Tee nuts.

These Tee nuts have little barbs that help keep them in place.

I am using this clamp to squeeze the Tee nuts into the drilled holes in the baffle.

 

Now we can begin wiring this unit up.  The black and white pair connect the two loudspeakers in parallel.

The red and black wires come from the power amp in this unit.  This Neutrik connector will disconnect the power amp when an external amp is connected to the cabinet at this port.  The soldering is done outside the unit.  BTW this is a Neutrik NL4MD-V-S.  The mating connector, an NL4FX, was supplied to the customer for his own wiring.

That looks pretty nice, in spite of the marks in the Tolex.

The new loudspeakers are in place and this unit is ready to test!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626